1. Field of the Invention
This invention relates to digital transmission systems and, more particularly, to supervisory signaling in such digital transmission systems.
2. Description of the Prior Art
It is well-known to utilize digital transmission techniques in the telephone plant. Digital systems of this type, using pulse code modulation (PCM), have been used in interoffice trunks to accommodate 24 voice frequency channels on two pairs of twisted wires. Such a transmission system, called the D3 channel bank, is disclosed in J. H. Green et al U.S. Pat. No. 4,059,731, issued Nov. 22, 1977.
Digital transmission systems must provide extra bits for forming and supervising the overall digital signal. The system described in the Green et al patent utilizes a "pulse borrowing" technique in which the least significant bit of every sixth PCM word in each channel is utilized to transmit supervisory signaling information for the associated channels. The slight degradation of the voice signal resulting therefrom is virtually unnoticeable and yet the arrangement provides a supervisory signaling channel for each voice channel which is adequate for standard supervisory signals (off-hook, dial pulsing, etc.).
The digital pulse stream for such digital transmission systems consists of 24 eight-bit words and one framing bit for each frame of the digital signal. Each of the 24 data words is a PCM representation of a voice frequency signal and the least significant bit (i.e., the eighth bit) of each channel is periodically dedicated for signaling purposes. That is, every sixth frame includes supervisory signaling bit positions in the eighth bit position of each word. In addition, a framing bit position (once each frame) contains a framing pattern (in successive frames) which is used to frame the overall digital pulse stream and thereby permit the recovery of the supervisory bits as well as the PCM words.
The supervisory framing pattern in the Green et al patent uses framing bits in alternate frames to transmit a succession of three zeros followed by three ones, followed by three zones, etc. Two supervisory frames can be identified in such a framing scheme, one following the transition of the framing pattern from a zero to a one (the A bit) and one following the transition of the framing pattern from a one to a zero (the B bit). The A and B bits thus derived can be used to transmit continuous supervisory states by forcing either of these bit positions to contain continuous one or continuous zeros. The four possible combinations of the two-state A and B bits in these two positions are used to represent four unique and distinct supervisory states.
In prior art applications of such a digital transmission system, the A and B bits are used to represent supervisory states which must be transmitted between telephone central offices in order to control the setting up and taking down of the telephone connections. The four possible supervisory states were adequate for this purpose.
If such a digital transmission system is utilized in the subscriber loop plant, it is possible to take advantage of the same digital circuit technology and, indeed, many of the same digital circuits themselves for this application. Unfortunately, however, transmission in the subscriber loop plant requires a much larger number of supervisory states. Multiparty ringing signals, for example, and coin station control signals involve a larger number of supervisory states. While it is possible to derive such a larger number of supervisory states by borrowing additional voice bits, this arrangement has the disadvantage of further impairing the voice signals. Another proposal, i.e., substituting a data channel for the signaling bits, produces a nonstandard signal format and thereby introduces a basic incompatibility between the existing digital network and the subscriber loop applications of the digital T-type transmission systems.